The present invention relates to blocked polyisocyanate crosslinking agents, to the production thereof and to the use thereof in single-component stoving lacquers, particularly OEM clear lacquers (original equipment material, initial lacquer coating).
As is known, blocked polyisocyanates are used as additives which impart improvements to single-component stoving lacquers which are used in the lacquer coating of automobiles. The stringent requirements of improved lacquer technology properties can be fulfilled relatively satisfactorily by blocked polyisocyanates. Other properties, however, such as a low stoving temperature up to a maximum of 140xc2x0 C., or reduced thermal yellowing when over-stoved, cannot be achieved with all the raw materials which are available.
The most important industrial lacquer polyisocyanates are based on 1,6-diisocyanatohexane (HDI). The available blocking agents for polyisocyanates on an industrial scale when maximum deblocking temperatures of 140xc2x0 C. are required are malonic esters and 1,2,4-triazole. A combination of HDI-based polyisocyanates with malonic esters or 1,2,4-triazole, or, as shown in the comparative example given later, with a malonic ester/1,2,4-triazole mixture, does not result in a usable liquid lacquer crosslinking agent which is stable on storage, because these products crystallise out. The tendency of HDI polyisocyanates blocked with malonic ester to crystallise is already known from EP-A 600 314, and that of triazole-blocked polyisocyanates is known from EP-A 654 490 and 741 157.
The object of the present invention was to modify polyisocyanates which are based on HDI and which are blocked with mixtures of malonic ester and 1,2,4-triazole, in order to form liquid crosslinking agents, which do not crystallise in solvents, for single-component stoving lacquers.
It has been possible to achieve this object with the blocked polyisocyanates described below.
The present invention relates to aliphatic and/or cycloaliphatic polyisocyanates, wherein
A) 5-95, preferably 70 to 30, equivalent % of the isocyanate groups thereof are blocked (reacted) with an acidic CH ester,
B) 5-95, preferably 70 to 30, equivalent % of the isocyanate groups thereof are blocked (reacted) with 1,2,4-triazole, and
C) 0-10 equivalent % of the isocyanate groups thereof are blocked (reacted) with other blocking agents which are different from A) and B),
characterised in that they contain 0.05-1.0 moles of formaldehyde in incorporated form with respect to 100 equivalent % of blocked NCO groups.
The present invention also relates to a method of producing the non-crystallising blocked polyisocyanates according to the invention from
a) 100 equivalent % of aliphatic and/or cycloaliphatic polyisocyanate components,
b) 5-95, preferably 70 to 30, equivalent % of a blocking agent from the group comprising acidic CH esters,
c) 5-95, preferably 70 to 30, equivalent % of 1,2,4-triazole,
d) 0-10 equivalent % of a blocking agent which is different from b) and c), and
e) 0.05-1.0 moles of formaldehyde,
characterised in that the formaldehyde is added with catalytic amounts of a base after complete reaction of the NCO groups present, and is reacted out.
The non-crystallising, blocked polyisocyanates according to the invention preferably have a content of stabilisers against thermal yellowing amounting to
0.1-5.0% by weight of hydrazides comprising a structural unit of formula (I) 
xe2x80x83and
0.1-5.0% by weight of xe2x80x9cHALS aminesxe2x80x9d comprising a structural unit of formula (II). 
Stabilisers of this type are known from EP-A-829 500, for example.
Finally, the present invention relates to the use of the blocked polyisocyanates according to the invention as crosslinking agents for organic polyhydroxyl compounds in polyurethane stoving lacquers, for example for automobile clear lacquers.
A content of formaldehyde is an essential feature of the invention for the claimed polyisocyanate crosslinking agents. Surprisingly, formaldehyde exhibits two modes of action:
Firstly, it prevents crystallisation of the blocked polyisocyanates, particularly if the polyisocyanate component is based on HDI. Secondly, it increases the resistance of the blocked polyisocyanates to thermal yellowing.
0.1 to 0.4 moles paraformaldehyde are preferably used per 100 equivalent % of blocked polyisocyanate, and are incorporated by reaction by means of a catalyst such as Na methylate, for example. This is equivalent to about 0.8 to 5.0% by weight of formaldehyde with respect to the blocked polyisocyanate. A formaldehyde content of 2.0 to 3.0% by weight with respect to the blocked polyisocyanate is most particularly preferred.
The method according to the invention is carried out as follows:
The polyisocyanate component is placed in a reaction vessel, optionally together with a little solvent, e.g. 1-methoxy-2-propyl acetate (MPA), at about 50xc2x0 C., the acidic CH ester, e.g. malonic ester, is added thereto as a mixture with about 2% of a 30% Na methylate solution and is reacted at 70-90xc2x0 C. to give the calculated NCO content. Thereafter, the calculated amount of 1,2,4-triazole is added, dissolved in further MPA, and is reacted out at about 90xc2x0 C. so that NCO can no longer be detected. The batch is cooled to about 45xc2x0 C. and the clear solution of paraformaldehyde, which is about 3% with respect to paraformaldehyde, is added to a mixture comprising 30% Na methylate solution as a base and isobutanol, and is subsequently stirred at 45xc2x0 C. for about 1 hour. The formaldehyde is incorporated in the course of this procedure; the content of free formaldehyde is at the analytical detection limit. About 0.5%, with respect to the batch, of dibutyl phosphate is added thereto and the pH is adjusted to about 7.5-7.0 therewith. According to one particular embodiment of the invention, other stabilisers against thermal yellowing can also be added in addition to this clear solution.
The polyisocyanates on which the blocked polyisocyanates according to the invention are based are known lacquer polyisocyanates which comprise aliphatically and/or cycloaliphatically and/or aromatically bonded isocyanate groups and which have an isocyanate content of 7 to 30, preferably 12 to 25% by weight.
The polyisocyanates which are preferably used are lacquer polyisocyanates known in the art, which comprise biuret, isocyanurate, urethane, allophanate and/or uretdione groups and which are based on 1,6-diisocyanatohexane (HDI), 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane (IPDI) and/or bis-(4-isocyanatocyclohexyl)-methane (H1 2MDI or Desmodur W(copyright)/Bayer AG). In addition, lacquer polyisocyanates which are trimerised asymmetrically, and which therefore comprise iminooxadiazinedione groups in addition to isocyanurate, and which are based on 1,6-diisocyanatohexane can also be used, such as those described in EP-A 798 299 for example.
Polyisocyanates based on other isocyanates can also be used, such as those based on diisocyanatobutane-1,4, 2,4- or 2,6-diisocyanato-1-methyl-cyclohexane, 2,5- and/or 2,6-bis-isocyanato-norbornane, 3- and/or 4-isocyanatomethyl-1-methylcyclohexane, 1,4-bis-(2-isocyanato-prop-2-yl)-benzene, 1,3-diisocyanatomethylbenzene, or 1,3- or 1,4-bis-isocyanatomethylcyclohexane, for example.
The substances which are most particularly preferred as the polyisocyanate component, however, are polyisocyanates which comprise isocyanurate and which are based on 1,6-diisocyanatohexane.
The substances which are used as the blocking component firstly comprise acidic CH esters, for example diethyl malonate or ethyl acetate and/or methyl, isopropyl, isobutyl or tert.-butyl derivatives thereof. Diethyl malonate is preferred. Secondly, 1,2,4-triazole is used in said blocking mixture, 1,2,4-triazole is produced on a large scale industrially from 1 mole of hydrazine hydrate and at least 2 moles of formamide. The acidic CH ester and triazole can each be used in amounts of 5-95 equivalent %, preferably 20-80 equivalent %.
The following substances can be used in conjunction as further blocking agents in amounts up to 50 equivalent %, preferably up to 20 equivalent %: oximes, such as butanone oxime for example, secondary amines e.g. diisopropylamine, acidic NH heterocycles, such as imidazole or 3,5-dimethylpyrazole for example, or lactams e.g. xcex5-caprolactam.
The stabilisers which are known from EP-A 0 829 500 can be used as additional stabilising components. The substance which is preferred in this connection is the hydrazide of formula (Ia) 
which is readily obtainable by the addition of hydrazine hydrate to 2 moles of propylene carbonate, and the HALS amine Tinuvin 770 DF(copyright) (manufactured by Novartis) which is not substituted at the N atom. 3 to 6% by weight of the former stabiliser is preferably used, and 0.5 to 1.5% by weight of the latter stabiliser is preferably used, with respect to the blocked polyisocyanate in each case.
The solvents which can be used are those which are known from polyurethane chemistry and which are inert to NCO groups, for example 1-methoxy-2-propyl acetate, solvent naphtha 100 or butyl acetate. If the NCO groups are blocked, alcohols e.g. isobutanol are also preferably used in order to facilitate transesterification with the alcohol residues of the blocking agents.
The advantages of the polyisocyanate crosslinking agents according to the invention, namely their good shelf life, i.e. no crystallisation, reduced thermal yellowing and good lacquer technology properties, are explained with reference to the following examples.